Reduced engine operation times in hybrid vehicles enable fuel economy and reduced fuel emissions benefits. However, the shorter engine operation times can lead to insufficient purging of fuel vapors from the vehicle's emission control system as well as insufficient time for completion of a fuel system leak diagnostics operation. To address some of these issues, hybrid vehicles may include a fuel tank isolation valve (FTIV) between a fuel tank and a hydrocarbon canister of the emission system to limit the amount of fuel vapors absorbed in the canister. An opening or closing of the FTIV may then be adjusted based on fuel system conditions to enable fuel vapor purging or leak diagnostics.
One example approach for fuel system control is shown by Fujimoto et al. in US 2003/0183206. Therein, when conditions for performing a leak diagnostics routine exist, the fuel tank isolation valve is closed while a canister purge rate is varied between a low purge rate and a high purge rate. A change in fuel tank pressure between the high canister purge rate condition and the low canister purge rate condition is used to infer fuel system degradation.
However, the inventors herein have identified potential issues with such an approach. As one example, fuel vapor purging operations may compete with the leak diagnostics routine for available time during the vehicle drive cycle. In other words, while the (higher and lower) purge rates may be sufficient to enable fuel system degradation to be identified, the duration of purging may not be long enough to enable the canister to be sufficiently purged. As a result, during a subsequent drive cycle, fuel vapors may not be stored and exhaust emissions may be degraded. On the other hand, if the purging operation is allowed to continue to empty the stored fuel vapors, there may not be enough drive cycle time left to perform the leak detection routine. As a result, fuel system degradation may not be timely determined and exhaust emissions may again get degraded.
In one example, some of the above issues may be at least partly addressed by a method of operating a fuel system including a fuel tank coupled to a fuel vapor canister via an isolation valve. The method may comprise purging fuel vapors from the canister to an engine intake for a duration with the isolation valve open until a threshold level of fuel tank vacuum is generated. In this way, the vacuum generation potential of a purging operation can be advantageously used to generate the vacuum required for a leak detection routine.
In one example, when purging conditions are met, and when a purge flow rate (as determined based on a canister load and the engine speed-load conditions) is higher than a threshold rate, it may be determined that a purging operation has vacuum generation potential. If there is insufficient fuel tank vacuum for performing a leak detection diagnostic routine (e.g., the fuel tank vacuum level is lower than a target level), the purging may be performed with the isolation valve open for a duration until the target level of vacuum is attained. Once the target fuel tank vacuum is achieved, the isolation valve may be closed to isolate the fuel tank and initiate a leak detection routine. For example, a bleed up rate of the fuel tank vacuum may be monitored to identify a fuel tank leak. Optionally, the purging may be continued with the isolation valve closed such that fuel vapor purging to the engine intake and fuel tank leak detection are performed simultaneously.
In this way, by purging fuel vapors from a canister with an isolation valve open for at least a duration of the purging, fuel vapor purging may be opportunistically used to reduce a fuel tank pressure to a desired vacuum level, such as a vacuum level at which a pressure decay based leak diagnostics routine can be performed. Thereafter, by purging with the isolation valve closed while a leak detection routine is performed, both fuel vapor purging and leak diagnostics can be performed and completed within the same vehicle drive cycle. In addition, cycle to cycle variation in test results may be reduced. By improving the completion frequency of both purging and leak detection operations, emissions compliance may also be better ensured.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.